Apr. 5, 1924 
Anatom y of the Apple Maggot 
9 
base of the true head. Most of the head forms the larval mouth cavity and a 
part of the larval pharynx. 
An examination of figure 3 will show in a very simple, diagrammatic way 
just what has taken place in the evolution of the maggot's head and how the 
parts have come to be as they are. The head of the adult fly is formed by a 
restoration of the invaginated parts to their normal positions, and by the devel¬ 
opment of the mouth appendages from buds in their proper morphological 
places. The reader should note here that the usual diagrams of the metamor¬ 
phosis of the muscoid head, taken from Korscheldt and Heider (22), are incor¬ 
rect in that they represent the frontal sacs as arising from the rear end of the 
larval pharynx and not from its anterior end. Such a scheme leaves nothing to 
account for the anterior parts of the head invaginated in advance of the basis 
of the frontal sacs. 
The structure of the pharynx and its appendages in the larva of Rhagoletis is 
shown in Plate 3, C. The pharynx itself is a wide passageway from the mouth, 
Fig. 3.— Showing the changes that have taken place in the head of a fly larva during evolution from the 
primitive condition of lower forms (A) to the complex structure of higher forms (B). A, diagrammatic 
lengthwise section of head and prothorax of larva of a lower fly, such as Chironomus, with external mouth 
parts {Lm, Lb) and with a small larval antenna (L Ant), but with a long imaginal antenna (IAnt) grow¬ 
ing within a frontal sac ( FS) or deep invagination of the hypoderm of the forming pupa {Pu) and with 
only its tip in the larval antenna; the pharynx {Phy) and oesophagus in normal position. B, a corre¬ 
sponding section through head and thorax of the maggot of a higher fly. The larval appendages are 
gone and the head has been turned in to form a new cavity, the atrium (Atr) in front of the pharynx 
{Phy), with a secondary evagination from its roof forming the bilobed dorsal pouch {DP) carrying the 
frontal sacs {FS) at the inner ends of its wings. The frontal sacs contain the imaginal rudiments or 
buds of the antennae (ah) and the compound eyes (ob). The labrum {Lm) is now buried behind the 
root of the dorsal pouch of the atrium, and the buds of the imaginal labium (LbB) arise beneath the 
floor of the atrium. The lips of the new mouth (x, x) are derived from the back of the original head 
{A, xx) and the larval head {LH) must be the primitive neck region. (Cf. PI. 3, F.) 
between the oral hooks ( Hk ), to the oesophagus ( OE ). Its floor ( VPhy) is a 
broad, convex membrane having a ribbed appearance externally ( d ) due to 
lengthwise ridges on its inner surface (PI. 3“, D, d). The lateral walls are strength¬ 
ened on each side by two plates, a small triangular one (PI. 3, C, A) in front and a 
much larger one ( B) behind. The first plate supports the oral hook of the same side 
by a loose articulation at its anterior angle. Its posterior angle is more or less 
fused with the second plate, and its ventral angle is connected by a transverse 
bridge ( e ) in the floor of the pharynx with the corresponding plate of the oppo¬ 
site side (PI. 3, E, e). A small rod from its upper edge (PL 3, C, a) helps sup¬ 
port the roof of the pharynx, which is further strengthened by a bridge plate 
(D) between the anterior ends of the triangular plates (A). 
The anterior part of the pharyngeal roof is convex and covers an anterior sec¬ 
tion of the pharyngeal cavity called the atrium by Wahl (51), shown diagram - 
matically (Atr) in figure 3, B. From the posterior end of the atrium the roof 
of the pharynx is produced upward as a dorsal pouch (PL 3, C, DP) having a 
